Why does my FPS drop from ~100 to ~50 after I "Build All Levels"?

Hello there!

I’m building a project in UE5, I get an average FPS of 100 normally, but when I “Build All Levels” it drops to around 50.

As soon as I move a light to reset the lighting, it shoots back up again.

I’ve tried this with lumen enabled and disabled. All my lights are set to stationary.

Is there something obvious I’m missing here?